The present invention relates to a disc brake with an integral parking brake.
As an example of a disc brake with an integral parking brake, there can be mentioned a disc brake disclosed in, for example, Japanese Patent Application Public Disclosure No. H9-250580, which comprises: a pair of pads disposed on opposite sides of a disc; a caliper including a cylinder having one end closed and a piston slidably fitted within the cylinder, wherein the piston is slidably moved under a brake fluid pressure introduced into an area between the cylinder and the piston to thereby press the pads against the disc; a cam mechanism adapted to be rotated on an axis perpendicular to the axis of the cylinder by operating a parking brake member, to thereby change an amount of projection of a cam rod; a push rod slidably supported in the cylinder and adapted to be slidably moved by being pressed by the cam rod of the cam mechanism; a clutch member threadably engaged with the push rod within the cylinder, which is slidably fitted within and abuts against the piston, wherein the clutch member is adapted to be pressed by the push rod, to thereby positively perform a slidable movement of the piston relative to the cylinder; a push rod biasing member disposed within the cylinder and adapted to bias the push rod towards the cam mechanism; and a spring cover disposed within the cylinder so as to hold the push rod biasing member between the push rod and the spring cover.
In this disc brake, when the parking brake member is operated, the amount of projection of the cam rod of the cam mechanism increases, to thereby move the push rod in the direction of the disc and also move the clutch member and the piston, together with the push rod, in the direction of the disc, thus mechanically pressing the pads against the disc.
In this disc brake in which a brake fluid pressure is introduced into the cylinder during normal braking, there are provided a piston seal for sealing a gap between an outer circumferential surface of the piston and an inner circumferential surface of the cylinder, a clutch member seal for sealing a gap between the piston and the clutch member, and a push rod seal for sealing a gap between the push rod and the cylinder.
When a brake fluid pressure is introduced into the cylinder in order to enable the piston to advance in the direction of the disc, the fluid pressure acts to thrust the piston in the direction of the disc. The clutch member, together with the piston, also thrusts in the direction of the disc until a built-in clearance provided between the clutch member and the push rod becomes zero (i.e., the clutch member engages the push rod).
If the piston continues to move in the direction of the disc even after the clearance becomes zero (when a clearance between the disc and the brake pad increases due to wear of the brake pad), abutment surfaces of the piston and the clutch member are separated from each other. In this instance, due to the fluid pressure acting on the clutch member through the clutch member seal provided between the piston and the clutch member, an axial force is generated, and the clutch member moves, while rotating, in the direction of the disc. Consequently, the abutment surfaces of the piston and the clutch member abut against each other again. This action is a so-called adjustment function which is performed to maintain a predetermined clearance between the disc and the brake pad.
If the brake fluid pressure is further increased and reaches a predetermined level, the clutch member is forced against the piston due to the brake fluid pressure acting on the clutch member. Therefore, the clutch member does not rotate (the adjustment function is not performed), and moves the push rod in the direction of the disc. Thus, overadjustment in the case of a high fluid pressure being applied to the cylinder can be prevented.
However, in the above-mentioned disc brake, when a high fluid pressure is applied to the cylinder, the push rod seal provided between the push rod and the cylinder receives a fluid pressure acting in a direction opposite to that of the movement of the piston pressed by the clutch member. Therefore, an output force of the piston decreases due to the fluid pressure acting on the push rod, thus limiting generation of an output force of the piston relative to the operation of the brake pedal. To compensate for such a loss of an output force of a piston, the diameter of a piston is required to be large, resulting in a large size of a disc brake.
Therefore, it is an object of the present invention to reduce the size of a disc brake.
The present invention provides a disc brake comprising: a pair of pads disposed on opposite sides of a disc; and a caliper including a cylinder having one end closed and a piston slidably fitted within the cylinder. The piston is adapted to be slidably moved, to thereby bring the pair of pads into contact with the disc. The disc brake also comprises: a cam mechanism adapted to be rotated to thereby change an amount of projection of a cam rod; a push rod disposed within the cylinder, which is adapted to be pressed and moved by the cam rod; and a clutch member disposed within the cylinder, which abuts against the piston while being threadably engaged with the push rod. The clutch member is adapted to be pressed by the push rod, to thereby positively perform a slidable movement of the piston relative to the cylinder. A push rod biasing member is disposed within the cylinder so as to bias the push rod towards the cam mechanism. Further, a spring cover is disposed within the cylinder, with the push rod biasing member being held between the push rod and the spring cover. In this disc brake, the push rod, the push rod biasing member and the spring cover form an assembly in the form of a single cartridge. The push rod comprises a front body portion threadably engaged with the clutch member and a rear body portion engaged with the cam rod. The front body portion and the rear body portion are separable from each other, with parting surfaces thereof existing in the interior of the cartridge. A radially outer portion of the front body portion of the push rod includes a rotation-preventing portion, which is adapted to restrict rotation of the front body portion and the cylinder relative to each other in a circumferential direction of the cylinder.
The present invention is advantageous in the following point. When a brake fluid pressure is introduced into the cylinder so as to effect advancement of the piston, the fluid pressure acts on the piston, to thereby generate, in the piston, a thrust force acting in the direction of the disc. In this instance, the fluid pressure also acts on the clutch member, so that a thrust force acting in the direction of the disc is also generated in the clutch member. In an initial period of a braking operation, the clutch member axially moves by an amount corresponding to a clearance for engagement provided between the clutch member and the front body portion of the push rod, thus pressing the piston. When the brake fluid pressure is further increased and reaches a predetermined level, the clutch member is forced against the piston due to the fluid pressure acting on the clutch member. Thus, the fluid pressure acts on the piston and the clutch member, so that a thrust force acting in the direction of the disc is generated in the piston and the clutch member. In this instance, the fluid pressure also acts on the rear body portion of the push rod, so that a thrust force acting in a direction opposite to the location of the disc is generated in the rear body portion of the push rod. In the present invention, the push rod is divided into the front body portion and the rear body portion which are separable from each other. Therefore, a clearance is formed between the parting surfaces of the front body portion and the rear body portion, and the thrust force of the rear body portion acting in the direction opposite to the location of the disc can be separated from the thrust force of the front body portion acting in the direction of the disc.